1. Field of the Invention
The present invention relates to an original reading apparatus for reading an image of an original.
2. Description of the Related Art
An image reading apparatus reads an image of an original and converts read information into digital data. The image reading apparatus can read a monochrome image and a color image. The image reading apparatus can be provided in an image forming apparatus, such as a copying machine and a facsimile machine. FIG. 9 is a diagram illustrating a flow of image signals in a conventional image reading apparatus.
A line image sensor 200 includes a light receiving portion 207 and an output amplifier 203 serving as an output buffer unit for outputting a signal from the light receiving portion 207 to the outside. The light receiving portion 207 includes a light receiving portion 207R provided with a red color (R) filter, a light receiving portion 207G provided with a green color (G) filter, and a light receiving portion 207B provided with a blue color (B) filter. The output amplifier 203 included in the line image sensor 200 is electrically connected to an AD converter with variable gain circuit 213. The AD converter with variable gain circuit 213 includes a variable gain circuit 208 and an AD converter 209. The AD converter with variable gain circuit 213 is electrically connected to a shading correction circuit 210. The shading correction circuit 210 corrects unevenness in digital output signals in a main scanning direction caused by fluctuations in characteristic of optical systems for illuminating an original and fluctuations in characteristic of each pixel of the line image sensor.
How to adjust the variable gain circuit 208 before reading an original will be described. In general, the gain of the variable gain circuit 208 is adjusted so that analog output signals of the line image sensor 200 may fall within an allowable input range of the AD converter 209. The reason will be described below. The following description is an example in which the AD converter 209 has an allowable input range of from 0 V to 1 V with a resolution of 10 bits. When the gain of the variable gain circuit 208 is 1 and the range of the analog output signals output from the output amplifier 203 is from 0 V to 1 V, the resolution of 10 bits (=1,024 gray levels) can be obtained. However, when the range of the analog output signals output from the output amplifier 203 is from 0 V to 0.5 V, the resolution is reduced to 9 bits (=512 gray levels). In this case, the resolution of 10 bits can be acquired by setting the gain of the variable gain circuit 208 to 2 to expand the input range of the AD converter 209 to a range of from 0 V to 1 V. In an actual procedure, a gain adjustment operation for the variable gain circuit 208 is carried out before the operation of reading an original. In the gain adjustment operation, the gain of the variable gain circuit 208 is set to 1, and a white reference plate is read. The white reference plate is a reference for a highest brightness and provided in the image reading apparatus. If the analog output signal input to the variable gain circuit 208 is below 1 V, the reciprocal of the analog output signal is set as the gain. For example, if the analog output signal is 0.75 V, the gain of the variable gain circuit 208 is set to 4/3. In this way, image data can be read with the most significant bit of the AD converter 209.
FIG. 10 is a graph showing the levels of analog output signals and digital output signals with respect to a pixel position in the main scanning direction. Referring to FIG. 10, shading correction to be carried out by the shading correction circuit 210 will be described. The shading correction is performed for correcting unevenness in digital output signals in the main scanning direction (line direction) caused by fluctuations in characteristic of optical systems (not shown) for illuminating an original to form an image and fluctuations in characteristic of each pixel of the line image sensor 200. In FIG. 10, the horizontal axis represents the pixel position in the main scanning direction of the line image sensor 200, and the vertical axis represents the levels of the analog output signals of the line image sensor 200 and the levels of the digital output signals converted by the AD converter 209. In FIG. 10, curves 301, 302, and 303 represent profiles of the analog output signals which are output from the light receiving portions 207R, 207G, and 207B, respectively, when the white reference plate is read. In this case, the gain of the variable gain circuit 208 is set to 1. Curves 304, 305, and 306 represent profiles of the analog output signals from the light receiving portions 207R, 207G, and 207B, respectively, which have been amplified by the variable gain circuit 208 in which the gain is adjusted to satisfy the most significant bit of the AD converter 209.
As can be understood from the curves 304 to 306 of FIG. 10, the levels of the analog output signals in the line direction of the line image sensor 200 are not uniform even though the white reference plate having uniform brightness in the main scanning direction has been read. The non-uniformity results from unevenness in imaging systems, optical systems, and sensor systems in the main scanning direction. In order to correct the non-uniformity, the shading correction circuit 210 calculates a digital gain for each pixel so as to obtain a digital output signal of a predetermined value (digital output target value). The digital output target value (shading target) to be set for calculating the digital gain is 1,023. The gain for obtaining the digital output signal of 1,023 is calculated for each pixel. As a result, when the white reference plate is read, the digital output signals output from the shading correction circuit 210 can be made uniform in the line direction as indicated by solid lines 307, 308, and 309.
Also in a monochrome sensor, the same as described in the above-mentioned color line image sensor is applied. Regarding analog signals of the monochrome sensor, the gain of an external variable gain circuit is adjusted so that the analog signals may take a value close to an allowable input range of an AD converter. After that, digital output signals from the AD converter are corrected by a shading correction circuit so as to be uniform in the main scanning direction (Japanese Patent Application Laid-Open No. H09-65121).
In the conventional technology, as illustrated in FIG. 9, in a path (transmission line) between the line image sensor 200 and the AD converter with variable gain circuit 213, a disturbance noise component N may be superimposed on the analog output signal (image signal component S) output from the output amplifier 203. In this case, a signal component to be input to the variable gain circuit 208 is the image signal component S with the disturbance noise component N added. When the gain of the variable gain circuit 208 is represented by A (where A>1), a signal component amplified by the variable gain circuit 208 before the AD converter 209 is A×(S+N)=A×S+A×N. Therefore, there is a problem that the disturbance noise component N is also amplified by the gain A similarly to the image signal component S.